Work

A0538-66: A transient ultraluminous X-ray source with a relativistic outflow, 69,ms pulsations, and anomalous optical activity

Year: 2026

Type: article

Abstract: A0538-66 is a peculiar Be/X-ray binary in the Large Magellanic Cloud, hosting the fastest spinning neutron star of its class (sim69,ms) and showing the brightest optical bursts of any system of this type. We report four observations obtained near periastron in 2025, together with MAXI and REM monitoring, and a reanalysis of archival RXTE data. On 30 September 2025 the source reached L_ ̊m X ≈ 1.5 ,erg,s-1 (0.2-10,keV), the highest level since 1980, and in February, May, and August 2026, MAXI detected similarly bright outbursts. During the =69.25878(2),ms and dot 10^ 39 observation taken on 30 September 2025, we detected the pulsations, measuring P_ ̊m spin P _ ̊m spin =6.58(9) ,s,s-1; the pulsed fraction was low ( ≈ 1.5 ,erg,s , and it makes >10^ -10 in 1$-10,keV), increased with energy, and the soft and hard pulse profiles were shifted by half a cycle. A new detection in RXTE data of 3 December 1998 gives a faster spin period of P=68.88813(28),ms. The spectra show emission lines from photoionised plasma at rest and blueshifted absorption lines that imply an outflow at ∼ 0.12c. This indicates a highly super-Eddington accretion rate and an intrinsic L_ ̊m X 10^ 39 -1 a transient ultra-luminous X-ray pulsar, a class of which only a few objects are known so far. Since 2023 the optical bursts observed by REM have become longer and brighter, resembling those of the 1980s, but most of them lack a luminous X-ray counterpart (L_ X 37 ,erg,s -1 ). We interpret these results in terms of a radiation pressure dominated accretion disc whose geometrically thick inner regions intermittently screen the innermost emitting regions.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202663088